SONET (Synchronous Optical Network) is a physical layer standard for fiber optic transmissions. SDH (Synchronous Digital Hierarchy--is an international standard of optical signals, payload formatting, and operations. Within the overheads of the various signal types that are defined within the SONET and SDH standards, there are bytes that are dedicated for use as bit interleaved parity checks (BIPs). By counting the number of BIP errors that are detected when such a signal is received, an estimate for the actual line error ratio can be obtained. While the correspondence between the number of BIP errors detected and the actual line bit error ratio has been documented, little (if any) guidance is presented as to how these relationships can actually be incorporated into an algorithm that facilitates the implementation of such a system.
The manner in which the signal degrade fault condition is monitored involves the integration of the number of BIP errors detected over time and comparing the result to a pre-determined threshold value. Should this number of errors exceed the threshold for the signal degrade fault condition, then it can be stated that the condition exists. A complication introduced by the SONET/SDH requirements is that they dictate the detection time for any line bit error ratio must be a factor of the actual bit error ratio, and not a factor of the signal degrade threshold that is being used. As such, it is necessary to monitor for multiple bit error ratios concurrently and make decisions concerning the presence (or absence) of signal degrade based on these results. This is typically accomplished by polling the received BIP error counts at a rate which is at least half of the required detection time for the highest bit error ratio to be monitored, and to maintain a history of these samples for a duration of time that is equal to the required detection time of the lowest bit error ratio.
The aforementioned technique is simple and effective but given the wide range of bit error ratios that must be monitored (1E-5 to 1E-10) and their associated detection times, the amount of memory needed to maintain a sample history of the appropriate length can get quite large. While this may not prove to be an issue for applications that have large memory resources, it can be paramount to those applications with smaller, fixed memory resources.
An object of the present invention is to address the issue of excessive memory usage that a typical application would require.